When a current flows through a superconducting coil around which a superconducting member is wound, magnetic flux lines of a magnetic field generated in the superconducting coil pass through the superconducting coil itself. For this reason, particularly, if a bismuth-based superconducting member is used, the amount of the flowing current is reduced as a magnetic flux density increases, and the current is difficult to flow. Accordingly, in view of the fact that, at a cryogenic temperature (liquid helium temperature), since the bismuth-based superconducting member has a much higher critical magnetic field than a metal-based material, it is proposed that the whole superconducting coil is cooled to the cryogenic temperature by liquid neon or liquid helium (for example, see Patent Document 1).
In such a superconducting coil apparatus, the amount of the current flowing through the superconducting coil can be maintained even when the magnetic flux density increases.    Patent Document 1: Japanese Patent Application, First Publication No. 2000-323321